The Third Generation Partnership Project group, known as 3GPP, is involved in ongoing standardisation work on the WCDMA group of protocols referred to as Universal Mobile Telecommunications System (UMTS) or 3G. A UMTS operator network can be separated into a number of major components, namely one or more core networks which are responsible for setting up and controlling user sessions, and a UMTS Radio Access Network (UTRAN) which controls access to the air interface. The architecture of a UTRAN is illustrated schematically in FIG. 1. The interface between the UTRAN and the user equipment (UE) is provided by nodes referred to as “NodeBs” (analogous to Base Stations in 2G/GSM networks). The NodeBs are responsible for transmitting and receiving data over the air interface and are controlled by Radio Network Controllers (RNCs). User and control data is routed between UEs and a core network via the NodeBs and the RNCs. The interface between a NodeB and an RNC is referred to as the Iub interface.
There are situations in which the same data may be transmitted between a given UE and an RNC via two or more NodeBs. This is referred to as Diversity Handover Function (DHO) or macro-diversity. The NodeBs may be controlled by the same or different RNCs. In the latter case, data is routed to the controlling (or serving) RNC via a drift RNC. The interface between the serving and the drift RNC is referred to as the Iur interface. Both scenarios are illustrated in FIG. 1.
The protocols responsible for carrying the payload between an RNC and a NodeB are described in 3GPP TS 25.435 and TS 25.427 for common (i.e. shared) and dedicated channels respectively. The protocol layers present at the RNC and NodeB are shown in FIG. 2. Of particular relevance here are the Frame Protocols (denoted FP in FIG. 2), which are responsible for carrying the user-plane data offered by upper layers (MAC/RLC) between the NodeB and the RNC.
The transport network (TN) underneath the Frame Protocol can be realized either as a cell-switched ATM network, or as a packet-based IP network. The typical approach to ensure that the transport network delivers the required service quality is to apply some kind of transport network admission control mechanism, which allows new connections as long as there is capacity available. This strategy works well for connections whose offered load and statistical properties are well known and understood, e.g. voice. The aggregated load of such connections can be easily and accurately estimated. If the estimated load exceeds the capacity of the transport network, no further connections are admitted. Thus, it can be ensured that all active connections receive the expected transport network service quality without wasting resources with an overly conservative admission mechanism.
Transport network reservation and admission control is much more difficult when packet switched (PS) data connections are considered, for a number of reasons:                The load presented by a PS channels can be much higher than for a voice connection: up to 384 kbps and more over a Dedicated Channel (DCH), and on the order of Mbps over a High Speed Downlink Shared Channel (HS-DSCH).        The traffic pattern over PS bearers shows a much higher degree of variation than for voice connections, having long idle periods followed by large data bursts.        The statistical properties of PS traffic are neither very well understood nor captured by any simple models. The load can be a complex function of link quality, pricing, customer segment, time of day/year, etc.        
When a transport network admission procedure is used for PS traffic, two different approaches may be adopted:
Prudent Admission:
To make sure that the transport network always delivers the desired performance, incoming connections are blocked at moderate reservation levels. The drawback is the likelihood of the unnecessary blocking of incoming connections at times when the admitted connections exhibit low activity. This solution leads to low utilization of transport network resources and blocked connections.
Generous Admission:
To avoid unnecessary blocking, more PS users are admitted than could momentarily be served if all connections turned active (the assumption being that not all users will simultaneously choose to send or receive data). The drawback is the increased likelihood of transport network overload at times when too many connections offer load.
Typically, the first of these approaches is used, meaning that only a few PS connections of, say 384 kbps, can be admitted at any given time. This is particularly true if the Iub is realized with thin E1 or T1 links.
It would be desirable to admit more PS connections (to avoid blocking) and have some method to handle the potential Iub overload conditions. There are two problems in implementing such a solutions. Firstly, there is no mechanism for explicitly detecting congestion on the Iub interface on a per connection basis. Secondly, the involved Iub protocols are unresponsive to Iub congestion. This means that the FP entity will always supply the transport network with the load offered by the MAC/RLC entities, irrespective of the potential overload over the Iub interface. Existing art on Iub load control (e.g. Saraydar et. al: “Impact of rate control on the capacity of an Iub link: Multiple service case, Proceedings WCNC 2003) employs centralised solutions based on some congestion control algorithm. EP1331768 and US2003223454 also propose centralised solutions to the problem of Iub load control.
The problem can be further illustrated by assuming a generous admission strategy where several 384 kbps bearers have been admitted over a thin Iub realization. When several/all bearers happen to offer traffic simultaneously, the result is likely to be delayed or lost Iub frames for some or all of the connections. Since the PS bearers are typically realised with Acknowledged Mode (AM), the receiving RLC entities will request re-transmissions of the lost frame content. This means that the overload may persist, as the FP instances will continue to shuffle data over the Iub so long as the sending MAC/RLC offers it. In a worst case scenario, no connection receives any data on time and all lost data goes to the sending RLC re-transmission buffers. The RLC/MAC/FP entities will then keep offering overload data to the Iub without any relief until protocol errors occur and the re-transmissions are abandoned with resets.